1. Field of the Invention
The present invention relates to a servomechanism for controlling an output in response to a demand signal by the substraction therefrom of a feedback signal and the application of the difference therebetween as the input to a load driving poor amplifier, and it is an object of the invention to provide a servomechanism which is protected, by being shut down, in the event of the loss of the feedback signal, or overloading of its output.
2. The Prior Art
While the invention is hereinafter described in relation to a servomechanism for positioning a read/write transducer in a disc data store, it is to be understood that this represents an example of and not a limitation on its use.
It is well knwon to employ a feedback servomechanism for radially positioning a read/write transducer over a rotating disc in a disc data store. The transducer is generally expensive, fragile, and intolerant of collision with the endstops which define the limits of its positioning range.
In the event of a failure of the feedback transducer signal, occasioned by such easily and cheaply remedied faults as a poor electrical connection thereto, the end of life of a light bulb, or the failure of a photocell, the power amplifier in the servomechanism responds only to the input demand signal, applies uncontrolled accelerating energy to the read/write transducer, and thereby necessitates the replacement of the read/write transducer subsequently to its destruction by single or repeated collision with its endstops.
While it is possible to arrange, in the operation of such a servomechanism, that the power amplifier should not remain in saturation for longer than a pedetermined period dictated by the frequency response and damping factor of the servomechanism, and to arrange to monitor the output of the power amplifier and to disable it should it remain in any one sense of saturation for longer than said predetermined period, there are circumstances where it is advantageous and desirable to arrange that the power amplifier should remain saturated for periods in excess of those consistent with servomechanism protection should the servomechanism happen to be out of control, and where such a scheme would be inappropriate. Such a set of circumstance arises when a head positioning servomechanism in a disc file is switched into velocity-controlled mode in order to execute the gross part of the movement of the head between data storage tracks. In order to minimise the transit time it is desirable to arrange that the power amplifier provides maximum accelerating and braking power to the load, thereby remaining near saturation for a time consistent with imparting a considerable potentially destructive velocity to the head. In order to prevent auto-induced velocity ringing in the load, it is also advantageous to arrange that the damping factor of the servomechanism is greater than unity, a state of affairs which also inhibits reversal of saturation polarity in the power amplifier.